The present invention relates to a liquid crystal panel that carries out image display by using the birefringence control system, and also concerns a liquid crystal display using such as liquid crystal panel.
Conventionally, the birefringence control system has been known as a method for controlling motion of liquid crystal molecules. In this system, a driving voltage is applied to liquid crystal molecules that are oriented perpendicularly to the substrate plane. Thus, the liquid crystal molecules are tilted in accordance with the driving voltage so that an image is displayed by controlling light transmission by using the resulting changes in birefringence.
In a liquid crystal panel used for such a liquid crystal display, in general, an opposing substrate, which is provided with a common electrode and an alignment film, and a pixel substrate, which is provided with a plurality of pixel electrodes and an alignment film, are arranged face to face with each other; a liquid crystal layer is sealed between these substrates; and a pair of polarizing plates are placed on the outside of these substrates.
In the liquid crystal panel of this type, in the case when the liquid crystal molecules 52 of the liquid crystal layer 53 are oriented in a completely perpendicular state with respect to the opposing substrate 51a or the pixel substrate 51b as illustrated in FIG. 14, the liquid crystal molecules tend to incline in random directions upon application of the driving voltage, with the result that the quality of displayed images is greatly reduced. For this reason, in a state in which no driving voltage is applied, the liquid crystal molecules are preliminarily tilted with a given angle (a pretilt angle). This makes it possible to preferably determine the oriented state of the liquid crystal molecules when the driving voltage is applied.
As one example of the liquid crystal display of this type, Japanese Laid-Open Patent Publication No. 151830/1990 (Tokukaihei 2-151830) discloses a liquid crystal display in which the orientation of a pretilt angle is set only either on the opposing substrate or on the pixel substrate so as to determine the oriented state of the liquid crystal molecules. Moreover, as examples of liquid crystal displays in which the orientation control for liquid crystal molecules is disclosed, those liquid crystal displays, obtained by a manufacturing method disclosed by Japanese Laid-Open Patent Publication No. 211424/1990 (Tokukaihei 2-211424), or disclosed by Japanese Laid-Open Patent Publication No. 107925/1991 (Tokukaihei 3-107925), have been known. In the former liquid crystal display, in addition to the perpendicular alignment process applied to the substrate, a rubbing treatment is further carried out so as to control the orientation of the liquid crystal molecules. In the latter liquid crystal display, a polyimide film is used as a perpendicular alignment film, and the liquid crystal molecules are oriented by rubbing the polyimide film.
In the conventional liquid crystal displays, it is proposed that the pretilt angle of the liquid crystal molecules be preferably set in the range of 0.5xc2x0 to 3xc2x0 with respect to the direction that is normal to the pixel substrate, and most preferably set in the range of 0.5xc2x0 to 1xc2x0.
Moreover, conventionally, in liquid crystal panels used for the above-mentioned liquid crystal display, a modification, such as minimizing regions except for the pixel electrodes, that is, for example, non-display sections located between the pixels, has been carried out. This is made so as to obtain brighter display images on the liquid crystal panel by making the pixel electrodes as large as possible and thereby maintaining a large effective display area. In this case, when the pixel electrodes are made as large as possible, the distance between the adjacent pixels is made smaller than or virtually the same as the cell thickness (the cell gap of the liquid crystal layer).
When the distance between the adjacent pixels is made smaller, the electric field exerted between the pixel electrodes in each pixel in the horizontal direction becomes stronger. The resulting electric field in the horizontal direction gives adverse effects on the orientation of liquid crystal molecules located between pixels, thereby causing a disorder in the orientation of the liquid crystal molecules. The disorder in the orientation of the liquid crystal molecules results in a reverse tilt region which makes the orientation direction of the liquid crystal molecules between the pixels reverse to the direction of those in other regions.
In the above-mentioned reverse tilt region, lines due to disclination, which are borders between different orientations of the liquid crystal molecules, take place on an image. In the region including these lines, the displayed image becomes dark, the luminance is reduced. Further, the display quality of the image itself decreases, merely providing rough, low-resolution images.
The above-mentioned adverse effects are particularly noticeable in high-precision liquid crystal panels using driving systems such as line inversion driving (V-inversion (Vertical-inversion) driving) and dot-inversion driving. In these cases, the entire liquid crystal panel becomes dark, and the display quality is excessively reduced.
Here, the application of the above-mentioned pretilt angle to the liquid crystal molecules allows the orientation of the liquid crystal to avoid the disorder due to the horizontal electric field, thereby making it possible to suppress the occurrence of the reverse tilt region. Consequently, it is possible to prevent the occurrence of disclination in a wider region, and also to avoid the above-mentioned adverse effects.
However, the condition for providing a pretilt angle in the range of 0.5xc2x0 to 3xc2x0 as described earlier is only applied to cases in which the region of non-display sections is large to a certain degree. In other words, in cases where the aperture section, which is the effective display section, is not so large in each pixel that the region of non-display sections is appropriately provided, the region of disclination can be shielded by a black matrix, etc. that is a non-display section under the condition that the above-mentioned pretilt angle is satisfied.
Under the above-mentioned condition, since the region of disclination is not used for image display, the resulting display screen makes it possible to provide a liquid crystal display that has a sufficiently satisfactory contrast. However, when the non-display section is reduced as the aperture section of the pixel is increased, it becomes more difficult to shield the region having disordered orientation by the non-display section such as the black matrix.
In particular, in the case of liquid crystal panels of the reflection type utilizing polarizing light, the black matrix is formed on the opposing substrate side. This case requires a margin for allowing substrates to be bonded to each other, thereby requiring a wider black-matrix region. This arrangement raises no problem in shielding the region of disclination, but raises a problem in expanding the effective display area.
One of the objectives of the present invention is to suppress disclination from occurring in a wide region by setting a pretilt angle of liquid crystal molecules at an angle within a predetermined range so that a liquid crystal panel having a large effective display area is achieved.
The other objective of the present invention is to provide a liquid crystal display of the birefringence control system having high display quality by using the above-mentioned liquid crystal panel.
In order to achieve the above-mentioned objectives, the liquid crystal panel of the present invention, which is a liquid crystal panel of the birefringence control system, is provided with: an opposing substrate and a pixel substrate that are arranged face to face with each other; and a liquid crystal layer that is sealed between the opposing substrate and the pixel substrate, and the liquid crystal layer has an oriented state in which liquid crystal molecules that have a negative dielectric anisotropy incline by an angle in the range of 3xc2x0 to 10xc2x0 with respect to the direction that is normal to the pixel substrate.
In the above-mentioned arrangement, upon application of a driving voltage to the liquid crystal layer, since the pretilt angle, which is the initial tilt angle of the liquid crystal molecules, is larger than a conventional value (0.5xc2x0 to 3xc2x0), it is possible to suppress the occurrence of a reverse tilt region. Consequently, lines of disclination due to electric potentials exerted between the adjacent pixels are shifted toward the edges of each pixel. The shift of the lines of disclination suppresses the occurrence of disclination in a wide region, thereby increasing the effective reflectance of the pixels.
Moreover, since the pretilt angle greater than that of a conventional device allows the tilt angle of liquid crystal molecules to rapidly change, it is possible to increase the response speed of the liquid crystal molecules. Thus, the image-writing time on the liquid crystal panel can be shortened.
Therefore, it becomes possible to provide a liquid crystal display which has a high efficiency in utilization of incident light, can display brighter images, and enables a response at high speeds.